The invention relates to an arrangement for the coating of a running web.
Paper, cardboard etc. are normally produced from natural cellulose fiber. The paper production is a complicated process, which is started by the defibration of natural logs or other plant parts, fibre preparation and finally sheet formation. In the sheet formation one strives to distribute the fibers evenly as possible. Along with the dewatering, which is carried out by the sheet formation, the fibers are guided more near to each other and are linked, mainly through fluidum bonds.
The paper comprises fibres located in several layers, arbitrarily distributed. If one examines the paper through a magnifying glass, cavities are found between the fibers and the surface is found uneven--everything depending on the micro structure quoted above.
One might have different reasons for the desire to improve this uneven micro structure. One might have, for instance, the desire to reduce the porosity in the paper, in order to be able to apply it as packaging material. Or, it is also important to improve the paper surface itself, in order to be able to print on the paper with a good result. A common method for the surface levelling and the porosity reducing is the application of a layer on the top of the completed paper surface. A normal method is to mix finely divided pigment, for instance clay, with water along with some binding substance, for instance latex, and thereafter to supply this paste on the paper web. The surface smoothness is achieved by doctoring the paste by a trowel blade, a so called scraper knife. The doctored paper is hereafter dried, whereby the supplied paste is attached at the paper web through a certain penetration.
A scraper blade has been used for doctoring-levelling from the realization of the coating technique onwards. Since the dominating technique is based on the principle, in which the scraper knife doctors the excess amount of the paste, the motion pattern of the paste will become complicated in the gap between the scraper and the paper, especially because very high speeds are used in the coating. A normal method comprises the guiding of the paper web over a standstill, downstreams inclined scraper knife. Normal speeds are 600-1000 m/min, but even higher speed up to 2000 m/min are applied. The excess paste amount in the coating can be 10-20 ggr. This excess amount is forced in to the space between the scraper knife and the paper back-up drum, and leaves this space by flowing transversely relative to the knife, and is thereafter resupplied to a trough for a re-entry in to the machine bin. During the reflow over the knife, the re-entried paste meets that paste, which is guided along with the paper against the knife.
Several problems occur during the paste levelling by the up-to-now used "downstreams inclined" scraper knife, which are related to the complicated motion pattern at the gap between the knife and the back-up drum (paper). This reveales itself in rheology phenomena, that is, the paste alters characteristics. The viscosity is diminished or increased, a uncontrolled dewatering occurs, and so on. This might result in its being impossible to achieve an even paste coating, or that the paste is not levelled at the phase edge of the scraper knife, that is, a so-called "bleeding" occurs. The paste might also be dewatered soon under the phase edge of the scraper knife, as of which the dewatered pigment attached to the knife.